The present invention relates generally to phonograph pickups, and in particular to an improved phonograph pickup or electromagnetic cartridge for the reproduction of monaural, stereophonic and quadraphonic sound recordings.
In a typical cartridge, there is provided a support usually in the form of a cartridge housing on which is mounted a stylus cantilever-generator assembly which includes a supported stylus lever having the requisite system compliance such that the stylus thereof may track the usual 45.degree.-45.degree. groove of a record disc. An appropriate magnetic structure is provided including first and second pairs of coils, with each pair typically being connected in series. The pairs of coils have signals induced therein which are related to the modulations derived by the stylus from the record groove. Signals or voltages are induced in the coils through the provision of a ferrous generator which is disposed in proximity to magnetic gaps defined by the permanent magnet and pole pieces of the magnetic structure.
In the manufacture of pickups for sound reproduction from monaural, stereophonic and quadraphonic records, it is important for the combined stylus lever-generator system to accurately translate stylus motion to the generator without the introduction of spurious generator motions which result in distortion. If the particular suspension means employed for the generating system introduced displacements of the effective center of generator motion due to transmitted stylus motion, the undesired generator motion will manifest itself as improper signal reproduction resulting in distortion which will vary over the frequency response of the pickup. For example, if incident to a change in frequency or amplitude of stylus motion there is a tendency for the particular selected pivot point to wander, unwanted and unpredictable motion of the generator will result causing spurious signal generations (distortions) which will vary randomly over the frequency and amplitude range of the cartridge.
One popular type of pivot for the stylus in such cartridges in an O-ring suspension wherein the stylus is supported by the generator; and the generator is received within a stationary O-ring and appropriately anchored such that an effective stylus pivot can be considered to be located in the center plane of the O-ring. Using this type of O-ring suspension, the effective pivot will be displaced axially along the stylus lever, dependent upon the interaction forces of stylus and generating masses, stylus beam and dynamic amplitudes of the combined parts. It is virtually impossible to eliminate such shifting or indeed to predict the pattern thereof over the frequency range of a cartridge. It is generally understood that this will result in distortion.
In another popular type of pivot system, the stylus lever having a generator mounted thereon is anchored on an axial pivot secured to a base support, which axial pivot is preloaded by an elastomer placed in axial compression between the stylus lever-generator assembly and the base support. When in a static or neutral position, the pivot for this type of system is axially positioned betwen the generator and the base support. During operation, the interacting forces which cause the pivoting position to shift and result in spurious signal generations are potentially even more complex. In this particular configuration, spurious motions of the generator are minimized; but inherently there is a tendency for the generator to be displaced at right angles to the axial support providing the pivot. The foregoing shifting and rotation of the generator introduces offsetting moments on the pivot which manifest themselves as effective displacements of the pivot, introducing further distortions.
An additional effect which comes about as a result of shifts in the effective pivot for the stylus lever is the introduction of the stylus of rapidly changing dynamic mass effects. The shifting of the pivoting position causes the elastomeric materials to act as resistive elements, rather than to serve the intended function of solely providing the pivotal support, thereby resulting in an effective increase of dynamic mass at the stylus. Such increase in dynamic stylus mass manifests itself as a relatively large dynamic mass for a correspondingly small static mass and notwithstanding such small static mass, severely limits the frequency response and increases distortion parameters of the pickup. Restriction of such frequency response and increased distortion is detrimental to such pickup performance, particularly in recently introduced systems for the reproduction of quadraphonic sound.
Accordingly, I have invented a new and improved suspension system for an electromagnetic cartridge which separates the various undesirable elements and allows them to be controlled individually with a relatively high degree of accuracy. With such separate control, it is possible to determine and program the dynamic effects necessary for optimum mechanical performance. With my new system, displacement of the pivot of the stylus lever is substantially reduced and highly controlled regardless of the frequency or mass deviations in the system.